This invention relates to a cable equalizer for AES digital audio data.
When an electrical signal is transmitted over a cable from a transmitting end to a receiving end, frequency-dependent attenuation may cause the waveform of the signal at the receiving end of the cable to be significantly different from the waveform of the signal at the transmitting end. It is known to compensate for this frequency-dependent attenuation by equalizing the signal at the receiving end.
A typical form of automatic cable equalizer for equalizing a signal Vin at the receiving end of a cable 8 is shown schematically in FIG. 1. The equalizer includes an amplifier 10 having a transfer function H(s) and a mixer 14 which receives both the signal Vin and the output of the amplifier 10 and provides an output signal Vout. The transfer function H(s) is the mathematical inverse of the transfer function of a fixed length of the same cable material as is used in the cable 8. Accordingly, the amplifier 10 behaves as a high-frequency emphasis circuit which compensates for the loss of a fixed length of cable. The value of the mix coefficient xcex1 depends on the actual length of cable between the transmitter and the equalizer and is derived from the output signal Vout of the equalizer, by comparing a voltage parameter of the output signal Vout with a reference value Vref and adjusting xcex1 in order to minimize the difference between the values.
A more basic type of cable equalizer is shown schematically in FIG. 9. Referring to FIG. 9,
Vout=xcex1Vin+(1+xcex1)H(s)Vinxe2x80x83xe2x80x83(1)
H(s)=(1+Rf/Zc)
Substituting for H(s) in equation 1,
Vout=Vin+Vin(Rf/Zc(1xe2x88x92xcex1))xe2x80x83xe2x80x83(2)
Equation 2 is of the form
Vout=K1Vin+K2(1xe2x88x92xcex1)Vinxe2x80x83xe2x80x83(3)
where K1 and K2 are constants.
The equalizer shown in FIG. 9 is used in stages, depending on the cable length. A given equalizer might be designed to correct for frequency-dependent attenuation by 500 feet of cable, and if the cable length were 1000 feet, two equalizers of this design would be used. The mix coefficient enters the equation describing the operation of the equalizer shown in FIG. 9 through the number of stages of equalization that are employed.
Referring again to FIG. 1, if the peak amplitude of the transmitted signal is known, the mix coefficient can be derived by employing a peak detector 18 to measure the peak amplitude at the output of the equalizer and a differential amplifier 20 to subtract the measured value of the peak amplitude from the known value of the transmitted signal""s peak amplitude.
Standards promulgated for video equipment establish the peak voltage level of the video signal as either 1 V or 800 mv. It is straightforward to measure the peak amplitude of the received signal during the equalizing pulses and employ this measured peak value to control an equalizer having the topology shown in FIG. 1.
One form of NRZ digital data coding is known as bi-phase mark coding. In bi-phase mark coding, a signal epoch is divided into bit cells of duration xcfx84 by a clock, and each source data bit is represented by a 2-cell doublet. Each coding doublet begins, and therefore also ends, with a transition. A source data bit 1 generates a transition between the two cells of the doublet, whereas a source data bit zero does not. Thus, a source data bit zero is represented either by the doublet 00 or the doublet 11, while a source data bit one is represented either by the doublet 10 or the doublet 01.
The Audio Engineering Society/European Broadcasting Union data stream for digital audio data employs a bi-phase mark coded signal in which each audio sample is represented by a subframe containing 32 doublets. The first 4 doublets of the subframe constitute a preamble containing at least one occurrence of the 3-cell sequence 000 or 111, which violates the bi-phase mark coding. FIG. 2 shows by way of example one form of preamble followed by a sequence of source data bits 11001.
When the AES digital audio signal is transmitted over a lengthy cable, the reactive impedance of the cable may cause distortion of the signal so that the waveform of the signal at the receiving end of the cable is significantly different from the waveform of the signal at the transmitting end. Referring to FIG. 3, the signal represented by waveform A at the transmitting end may have the waveform B at the receiving end of the cable. In order to recover the audio data with a high degree of reliability, it is necessary to compensate for the frequency-dependent attenuation of the signal by equalizing the signal at the receiving end.
The standard that prescribes the format of the AES digital audio signal does not specify one or two discrete values of the signal amplitude but merely specifies that the amplitude of the signal at the receiving end of the cable must be in the range from 100 mV p-p to 10 V p-p. This wide range of amplitudes does not allow an equalizer having the topology shown in FIG. 1 to derive the mix coefficient a with a sufficient degree of precision simply on the basis of the amplitude of the output signal of the equalizer.
In accordance with the invention there is provided a method of equalizing an electrical signal that is propagated over a path from a transmitting end of the path, at which a signal composed of pulses of uniform amplitude within a bit cell is impressed on the path, to a receiving end of the path, said method comprising applying the signal at the receiving end of the transmitting path both to a frequency-dependent emphasizer and to a first port of a mixer, applying the output signal of the frequency-dependent emphasizer to a second port of the mixer, employing the mixer to combine the signals received at its first and second ports in accordance with the amplitude of a mixer control signal to generate a mixer output signal, generating a control signal from the mixer output signal, the control signal having an amplitude dependent on the absolute value of the derivative of the amplitude of the output signal of the mixer during an interval that starts after the beginning of a bit cell and ends before the end of the bit cell, and applying the control signal to the mixer as the mixer control signal.